Impression creep of Sn–40Pb–2·5Sb peritectic solder alloy

Abstract

The power law creep behaviour of the Sn–40Pb–2·5Sb peritectic solder alloy was investigated using an impression test apparatus. The tests were carried out under constant stress in the range 17 to 39 MPa and at temperatures in the range 296 to 363 K. Assuming a power law relationship between the impression velocity and stress, power law stress exponents in the range 1–3 were determined. Analysis of the data showed that for all loads and temperatures, the activation energy was stress independent with values in the range 51–56 kJ mol^-1. Based on the stress exponents obtained and activation energy data, it is proposed that grain boundary diffusion is the major mechanism for creep of the Sn–Pb–Sb peritectic alloy under these test conditions.     

Precipitation and age hardening behaviour of AI–0·2Ti–0·2Be alloy

Abstract

The effect of 0·2%Be on the aging behaviour and microstructure of Al–0·2% Ti alloy is investigated. It is shown that the addition of Be induces some age hardening in non age hardenable AI–0·2Ti alloy and significantly increases the precipitate density, indicating a Be enhanced nucleation for the precipitate phase.

MST/1250     

Study on low temperature brazing of magnesium alloy to aluminum alloy using Sn–xZn solders

In this article, a series of Sn–xZn solders are designed for joining Mg/Al dissimilar metals by low temperature brazing. The effect of Zn content in Sn–Zn solders on microstructure evolution and mechanical properties of the different brazed joints are investigated. The experimental results indicate that Sn–30Zn alloy is identified as the optimized solder. Al–Sn–Zn solid solutions form and disperse in the brazing zone of the Mg/Sn–30Zn/Al brazed joint, decreasing the risk of embrittlement of the brazed joint. The average shear strength of Mg/Sn–30Zn/Al brazed joint can reach 70.73 MPa. The joint fractures in the coarse blocky Mg₂Sn intermetallic phases in the center of the brazing zone.     

Thermodynamic assessment of Sb–Sn system

Abstract

The Sb–Sn system has been assessed in terms of thermodynamic models for the individual phases. Expressions for the Gibbs energy as a function of temperature and composition are obtained and both phase diagram and thermochemical data are calculated and found to be in good agreement with the experimental data. The compound phase SbSn is treated by means of a Bragg–Williams–Gorsky approach.

MST/421     

Effect of beryllium and calcium on aging behaviour of Al–0·75Mg–0·5Si alloy

Abstract

The effect of microadditions of Be and Ca on the aging behaviour of Al–0·75Mg–0·5Si alloy is investigated. It is shown that the addition of 0·1%Be significantly increases the hardening rate and the maximum hardness level attainable when the alloy is aged at various temperatures from room temperature to 300°C, while the addition of 0·2%Ca decreases both the hardening rate and the maximum hardness level attainable. Optical and scanning electron microscopical observations show a significantly higher precipitate density for the Be containing alloy and a slightly lower precipitate density for the Ca containing alloy when compared with the base Al–Mg–Si alloy. The results are consistent with an earlier kinetic study that indicated a Be enhanced nucleation rate for precipitation in the same alloy.

MST/936     

Effect of carbon on hot deformation behaviour and microstructural evolution of near-α titanium alloy Ti–5·6Al–4·8Sn–2Zr–1Mo–0·35Si–0·7Nd

Abstract

The hot deformation behaviour and microstructural evolution of a near-α titanium alloy (Ti–5·6Al–4·8Sn–2Zr–1Mo–0·35Si–0·7Nd) containing 0·06%C or 0·3%C with bimodal or Widmanstätten starting microstructures were investigated using isothermal compression test at strain rates of 0·01–10 s^?1 in the α+β or β regions. In the α+β region, both alloys exhibited continuous flow softening. The globularisation of transformed β structure or the recrystallisation of globular α phase took place, which was more remarkable in the 0·3%C alloy. In the β region, both alloys exhibited steady-state flow behaviour. Dynamic recrystallisation of the β phase occurred in the 0·06%C alloy, while was absent in the 0·3%C alloy. Due to the solution hardening of carbon atoms for the phases and the pinning effect of the carbides on grain boundary, the apparent activation energies of the 0·3%C alloy are higher than those of the 0·06%C alloy in the corresponding α+β or β phase regions.     

Creep fatigue crack growth in 0·5Cr–Mo–V steel

Abstract

An investigation is presented of crack growth in a normalised and tempered 0·5Cr–Mo–V steel under cyclic displacement controlled loading conditions at 565–600°C. A transition from fatigue to creep dominated behaviour was observed as the duration of the tensile dwell period in the cycle was increased. This change was a result of a progressive increase in the extent of crack tip grain boundary damage accumulation which, in the long dwell tests, was sufficient to give rise to crack extension directly. Time dependent crack propagation rates during the dwells of the long dwell tests were found to approach those determined for static load conditions. No evidence was found for a significant creep-fatigue interaction and it appears that overall crack growth rates are determined by crack tip oxidation and damage accumulation processes.

MST/756     

Creep deformation behavior of Sn–Zn solder alloys

Creep properties of three Sn–Zn solder alloys (Sn–9Zn, Sn–20Zn, and Sn–25Zn, wt%) were studied using the impression creep technique. Microstructural characteristics were examined using a scanning electron microscope. The alloys exhibited stress exponents of about 5.0. The activation energy for creep was calculated to be ~50–75 kJ/mol with a mean value of 66.3 kJ/mol. The likely creep mechanism was identified to be the low temperature viscous glide of dislocations.     

Physicochemical Properties of Sb, Sn, Zn, and Sb–Sn System

The physicochemical properties, viscosity, density, and surface tension, were measured using the discharge crucible method (DC) on five alloys of Sb–Sn. The performance of the DC method was demonstrated with measurements on pure metals Sb, Sn, Zn, and comparisons with the corresponding literature data. The results reported in this study are for Sb–Sn alloys containing (10, 20, 25, 50, and 75) at% of Sb at 550 K to 850 K. The results show that all the physicochemical properties decrease with decreasing temperature and increase with increasing Sb content in the alloy. The experimentally measured surface-tension values are compared with the Butler model. Several models for viscosity are compared and discussed.     

Effect of medium-density direct current on dendrites of directionally solidified Pb–50Sn alloy

This paper investigated the effect of coupling of direct current (DC) and pulling rate on dendrites and cooling behaviours of directionally solidified Pb–50Sn alloy. Experimental results indicated that the secondary dendritic arm spacing (SDAS) decreased and temperature gradient increased as increasing current densities. Moreover, temperature rise and SDAS under positive DC were higher than those under negative DC. It was speculated that Joule heating and electromigration were obviously induced by the present DC. The effect of DC on the microstructure and solidification parameters was weakened as the pulling rate increases. The coarsening rate reduced from tf^1/3 toward a value of tf^0.29 when DC of ±200?A?cm^–2 were applied. The refinement mechanism of SDAS was discussed.     

Some mechanical properties of Sn–3.5 Ag eutectic alloy at different temperatures

The eutectic alloy Sn–3.5 wt % Ag has been examined as one of the lead-free solder alloys. Microhardness tests as a function of temperature were performed to calculate the effective activation energy of the solder alloy Sn–Ag and compared with the pure elements Sn and Ag. Various creep parameters such as, exponent n_tr and the parameter in the transient creep stage and the values of the stress exponent n from the steady-state stage were calculated under different constant applied stresses at different working temperatures. The structure changes of the alloy were reported before and after creep test.     

Strengthening Sn60–Pb40 solder alloy by adding trace amount of La

The high temperature tensile strength of traditional Sn60–Pb40 solder alloy has been increased by 70% by adding trace amount of rare earth element La into it. Meanwhile, the thermal fatigue life of solder joints has been increased by two times. Such improvement is attributed to the modification effect of La on the microstructure of Sn60–Pb40 alloy.     

Failure analysis on gas pressure formed spherical domes of Pb–Sn eutectic alloy

Abstract

This paper addresses the issue of instability of deformation during gas pressure forming of superplastic sheets. With regard to fracture strain, the plastic behaviour of the spherical dome has been described in terms of the local effective stress and the effective strain. These quantities are equated to the uniaxial stress state. The limiting effective thickness strain is obtained utilising the relations between the strain rate sensitivity index and the fracture strain. The results are found to be in good agreement with the measured failure strains.     

Effect of Sb addition on the tensile deformation behavior of lead-free Sn–3.5Ag solder alloy

Tensile deformation behavior of Sn–3.5Ag and Sn–3.5Ag–1.5Sb alloys was investigated at temperatures ranging from 298 to 400 K, and strain rates ranging from 5 × 10⁻⁴ to 1 × 10⁻² s⁻¹. After melting and casting, the samples were rolled to sheets, from which tensile specimens were punched and pulled to fracture in uniaxial tension tests. Scanning electron microscopy (SEM) was used to study the microstructure and fracture surface of the samples. Addition of 1.5% Sb into the binary alloy resulted in an increase in both ultimate tensile strength (UTS) and ductility. The enhanced strength was attributed to the solid solution hardening effects of Sb in the Sn matrix. The improved ductility was, however, caused by the structural refinement which results in the higher strain rate hardening of the Sb-containing alloy. This was manifested by the higher strain rate sensitivity (SRS) indices (m) of 0.14–0.27, as compared to 0.11–0.20 found for the Sn–3.5Ag alloy.     

Investigation of nanoscale precipitates developed in Al–0·73Mg–0·45Si–0·34Cu–0·21Cr–0·20Fe alloy

Abstract

In the present work, the developed nanoscale precipitates in Al–0·73Mg–0·45Si–0·34Cu–0·21Cr–0·20Fe (wt-%) alloy have been investigated by means of differential scanning calorimetry, electron microscopy (SEM and TEM) and microhardness measurements (HV). The addition of Cu assists the formation of Q′ phase which positively changes the alloy strength. The precipitation of β′′ nanoparticles is followed by the precipitation of β′ and/or Q′ precipitates. Both coherent and semicoherent precipitates have a positive contribution to the strengthening of the alloy. The average activation energy associated with the precipitation of β(Mg₂Si)+Q phases is very close to that for Q″ and/or Q′ phase which suggests that the two phase precipitation might be characterised by the same mechanism. The reaction order of the precipitation processes suggests that β′ and/or Q′ precipitates grow radially; whereas, β-precipitates grow in the three directions.     

Characteristics of steady state deformation of an Al–0.1 Mg alloy at low temperatures

The present work aims to address the characteristics of steady state deformation, which determines the limit of grain refinement for a given material by severe plastic deformation. The focus is on low temperatures at which most deformation processing is conducted. Submicron grained Al–0.1 Mg alloy prepared by equal channel angular pressing was deformed by plane strain compression in a channel-die and rolling at a constant strain rate of 10^?2 s^?1 and at a range of temperatures from 77 to 473 K to various strains. Microstructures were characterized by electron backscatter imaging and EBSD in a FEGSEM. Grain refinement to the ECAP submicron structure occurred during deformation at cryogenic temperatures of 77–213 K, whereas coarsening took place during deformation at elevated temperatures. A steady state deformation was observed at all temperatures where a constant grain structure was developed and maintained upon further straining. The microstructural characteristics of steady state deformation and mechanism responsible for the establishment of the steady state are discussed.     

Effects of residual porosity on strength of Sn–Pb/Cu6 Sn5 particulate composites

Abstract

The effect of residual porosity on the flow stress of particulate composites was studied for a composite in which it is possible to create high porosity (>60%) by using particle coatings. The material was a eutectic composition Sn–Pb solder with additions of intermetaliic CU₆Sn₅ particles. It was found that porosity affects the strength of these composites much less than for non-composites, an effect attributed to particle bonding during solidification. At the same time, residual porosity was found to inhibit strongly or even to prevent hardening of this material. A micromechanical model is developed which simulates this behaviour over a wide range of porosities.

MST/1928     

Glass-formation region of ternary Sn–Sb–Se-based chalcogenide glasses

Tin-antimony-selenium (TAS)-based system belongs to the ternary chalcogenide compounds of IV-V-VI group owing to their heavy elemental masses, their glass formation region was assumed to be small comparing to their counterpart elements in the same group. However, there were rare published reports on their glass structure, while their glass boundary formation region was not yet reported. It was the aim of this paper to map their glass-forming region experimentally using XRD and validate it theoretically using the average co-ordination number, , and the fraction of the bond distributions, f, from chemical order model. Theoretically, it was validated that the glass formation was arrested between 2.4 and the fraction of Sn–Se bonds, f_Sn–Se < 44.5%. XRD analyses of 66-as-prepared samples revealed that the glass formation region was located within the predicted area that mapped in structural triangle.     

Brittle intergranular fracture at elevated temperatures in low–alloy steel

Abstract

Recent studies of stress-relief cracking in low-alloy steels have focused attention on a novel mode of brittle intergranular fracture which occurs at elevated temperatures (300–650°C) in hard, coarse–grained heat–affected–zone microstructures. Fracture initiates at stress concentrators such as sharp cracks or inclusions, and can propagate under static loading at rates of 10^?11?10^?5 ms^?1 to produce intergranular facets with very little associated plastic deformation. The stress-intensity parameter K has been used to characterize crack growth, and three regimes of behaviour have been observed: (i) a threshold region at growth rates of 10^?11?10^?10 m S^?1, (ii)a plateau region, in which growth rates are independent of K between 10^?10 and 10^?8 m S^?1, and (iii) a region of highly K-sensitive crack growth between 10^?8 and ^?5 m S^?1. Independent Auger electron spectroscopy analyses have demonstrated that sulphur segregates locally to the high-temperature crack tip, giving rise to the embrittlement of a limited area of grain boundary. Together with other presegregated solutes, this enables brittle fracture to occur at high temperature, and the transfer of sulphur to the crack tip controls the rate of crack growth. Two models describing crack-tip sulphur segregation are currently proposed. In the first model, a quantitative analysis demonstrates that the crack-tip stress field will drive undersize solute atoms such as sulphur to the physical crack tip. In the second, the intergranular crack is modelled as a sharp cavity. Grain-boundary sulphides which are exposed by cavity formation become unstable and dissolve, saturating the cavity surface with sulphur, which is then drawn into the tip as part of the cavity growth process.

MSTj77     

Constitution of AuSn–Pb–Sn partial ternary system

Abstract

Thermal analysis, metallography, and electron-probe microanalysis have been used to corroborate one of the conflicting constitutions of the AuSn–Pb–Sn partial ternary system. The phase equilibria consist of two ternary transition reactions at 13–0Au, 48–0Pb, 39–0Sn at.-%, 275°C and 8–5Au, 28–0Pb, 63–5Sn at.-%, 210°C, and a ternary eutectic at 3–5Au, 20–0Pb, 76–5Sn at.-%, 177°C. The solubility of Pb in the ternary system is substantial and between 275 and 177°C the system can dissolve up to 4 at.-% Au and 27 at.-%Sn. The maximum width of the AuSn primary phase field in the partial AuSn–Pb–Sn system is approximately 3 at.-%. and that of Sn approximately 2 at.-%.

MST/82